New 2D material could be used in electrochemical energy storage

A team of Scientists almost a decade ago predicted that boron atoms would cling too tightly to copper to form borophene, a flexible, metallic two-dimensional material with potential across electronics, energy, and catalysis.

Predicted by researchers at Rice University led by materials scientist Boris Yakobson, new research shows the prediction holds up, but in an unexpected way.

“Borophene is still a material at the brink of existence, and that makes any new fact about it important by pushing the envelope of our knowledge in materials, physics and electronics,” said Yakobson, Rice’s Karl F. Hasselmann Professor of Engineering and professor of materials science and nanoengineering and chemistry.

On copper, boron would bond too strongly

“Our very first theoretical analysis warned that on copper, boron would bond too strongly, and even if borophene did form, it would be hopelessly attached to the substrate. Now, more than a decade later, it turns out we were right ⎯ and the result is not borophene, but something else entirely.”

Researchers revealed that unlike systems such as graphene on copper, where atoms may diffuse into the substrate without forming a distinct alloy, the boron atoms in this case formed a defined 2D copper boride ⎯ a new compound with a distinct atomic structure. The finding sets the stage for further exploration of a relatively untapped class of 2D materials, according to researchers.

2D boron nanomaterials have attracted substantial interest

Published in Science Advances, the research reveals that since the first realization of borophene on Ag(111), two-dimensional (2D) boron nanomaterials have attracted substantial interest because of their polymorphic diversity and potential for hosting solid-state quantum phenomena.

“Here, we use atomic-resolution scanning tunneling microscopy (STM) and field-emission resonance (FER) spectroscopy to elucidate the structure and properties of atomically thin boron phases grown on Cu(111). Specifically, FER spectroscopy reveals charge transfer and electronic states that strongly differ from the decoupled borophene phases observed on silver, suggesting that the deposition of boron on copper results in strong covalent bonding characteristic of a 2D copper boride,” said researchers in the study.

Earlier, studies synthesized borophene on metals like silver and gold, but copper remained an open ⎯ and contested ⎯ case. Some studies also highlighted that the boron might form polymorphic borophene on copper, while others suggested it could phase-separate into borides or even nucleate into bulk crystals.

Researchers revealed that resolving these possibilities required a uniquely detailed investigation combining high-resolution imaging, spectroscopy and theoretical modeling.

Experimentalists saw spectroscopy signatures

Yakobson underlined that what experimentalists first saw were rich patterns of atomic resolution images and spectroscopy signatures, which required a lot of hard work of interpretation.

These efforts revealed a periodic zigzag superstructure and distinct electronic signatures, both of which deviated significantly from known borophene phases. A strong match between experimental data and theoretical simulations helped resolve a debate about the nature of the material that forms at the interface between the copper substrate and the near-vacuum environment of the growth chamber, according to a press release.

“2D copper boride is likely to be just one of many 2D metal borides that can be experimentally realized,” said Mark Hersam, Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University and a co-corresponding author on the study.

“We look forward to exploring this new family of 2D materials that have broad potential use in applications ranging from electrochemical energy storage to quantum information technology.”

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